Electromagnetic compensating balance

Abstract

An electromagnetic compensating balance for the measurement of forces having a permanent magnet circuit assembly with an annular enclosed housing and with a central core defining a narrow annular air gap between a portion of the housing and core, and having a movable bobbin concentrically arranged around the core with a coil located in the air gap, the bobbin having end portions projecting above and below the housing which are connected to leaf support members for independent support of the bobbin from the magnet assembly, the upper end portions being connected to a weighing pan for weighing samples. The balance has sensing means comprising a first pair of capacitor rings located above said coil and a second pair of capacitor rings located below said bobbin connected in a manner which eliminates canting effects in said bobbin from a reference position, said sensing means generating a transducer output voltage representing an analog to displacements in the bobbin, said output voltage controlling a servo system for generating a d.c. current through the coil which activates the coil to create a compensating equilibrium force by the bobbin opposing the force of the weighing sample, the servo system having circuit means for providing an analog voltage representing current level said analog voltage controlling an analog to digital measuring circuit which operates with said servo circuit means but is independent thereto.

What is claimed is: 
     
       1. An electromagnetic compensating balance device for measuring forces comprising: a stationary permanent magnetic circuit means for generating a magnetic field;   an electromagnetic coil means movably mounted in said stationary magnetic circuit means for displacement relative thereto, said coil means including structural means for orienting said coil means with respect to said permanent magnet means for limited linear displacement on a predetermined axis;   transducer means mounted partially on said magnet circuit means and partially on said coil means for detecting displacements of said coil means relative to said stationary magnet and circuit means and defining a reference position of said coil with respect to said permanent magnet means, said transducer means including first means for compensating for error producing dimensional changes in said coil structure means along said axis and second means for compensating for positional changes of said coil structure means skew to said axis and for local environmental changes;   transducer circuit means cooperating with said transducer means for generating a transducer output signal related to displacements of said coil means from said reference position;   servo circuit means controlled by said transducer output signal for generating a direct current signal for driving said coil means toward said reference position; and   coil circuit means connecting said servo circuit means and said coil means for applying said current signal to said coil and actuating said coil toward said reference position, said coil circuit means including circuit means for providing an analog output signal representative of the magnitude of said direct current signal.   
     
     
       2. The balance device of claim 1 comprising further measuring circuit means coupled to said analog output circuit means for measuring said analog output, and generating a readable output measurement of a directional force applied to said coil means. 
     
     
       3. The balance device of claim 2 wherein said analog output signal is a d.c. voltage signal and said measuring circuit means includes buffer means inhibiting interference with said analog output circuit means and said coil circuit means. 
     
     
       4. The balance device of claim 3 wherein said measuring circuit means includes an analog to digital converter. 
     
     
       5. An electromagnetic compensating balance device for measuring forces comprising: a stationary permanent magnetic circuit means for generating a magnetic field;   an electromagnetic coil means movably mounted in said stationary magnet circuit means for displacement relative thereto, said coil means being generally cylindrical with a central axis and said permanent magnet means surrounding said coil means coaxially with said axis;   structural support means interconnecting said permanent magnetic and coil means at both ends of said coil means for orienting and guiding said coil means along said axis;   transducer means comprising two sets of capacitor rings spaced apart axially of said coil at symmetrical positions with respect to the ends of the coil with each of said sets of capacitor rings including a first pair of rings mounted on said coil coaxial with said axis and a second pair of rings mounted on said permanent magnet coaxial with said axis and with the rings of one pair positioned axially of said coil between the rings of the other pair, said transducer means defining a reference position of said coil with respect to said permanent magnet means, whereby displacements of the coil means relative to the stationary magnet circuit means may be detected;   transducer circuit means cooperating with said transducer means for generating a transducer output signal related to displacements of said coil means from said reference position;   servo circuit means controlled by said transducer output signal for generating a direct current signal for driving said coil means toward said reference position and   coil circuit means connecting said servo circuit means and said coil means for applying said current signal to said coil and actuating said coil toward said reference position, said coil circuit means including circuit means for providing an analog output signal representative of the magnitude of said direct current signal.   
     
     
       6. An electromagnetic compensating balance device for measuring forces comprising: a stationary permanent magnet circuit assembly comprising an annular substantially enclosed outer housing and an inner concentric core wherein a narrow air gap with a central axis is defined between at least a portion of the housing and a portion of the core;   a movable annular bobbin member arranged between the housing and the core, the bobbin member having means connected thereto for supporting weighing samples external of said outer housing;   a coil means mounted on said bobbin member and situated substantially in said air gap for generating an electromagnetic field and actuating displacements in said bobbin;   transducer sensing means for detecting displacements of said bobbin member relative to said magnet assembly comprising: a first pair of annular capacitor means on said magnet assembly mounted toward one end of said axis from said air gap, said capacitor means on said assembly having cooperating capacitor means on said bobbin;   a second pair of annular capacitor means on said magnet assembly mounted toward the other end of said axis from said air gap, said capacitor means on said assembly having cooperating capacitor means on said bobbin wherein one of said annular capacitor means in said first pair is electrically connected to one of said annular capacitor means in said second pair and the other annular capacitor means in said first pair is electrically connected to the other annular capacitor means in said second pair;   transducer circuit means cooperating with said sensing means for generating a transducer output signal related to displacements of said bobbin member;   drive circuit means controlled by said transducer output signal and connected to said coil means for generating an operating current and activating said bobbin member to an equilibrium position along said axis opposing the force of the weighing sample; and   measuring circuit means operable cooperatively with said drive circuit means for measuring signals representative of the force of the weighing sample.     
     
     
       7. The balance device of claim 6 wherein said drive circuit means comprises, servo circuit means for generating a direct current signal related to said transducer output signal to generate a compensating electromagnetic force in said coil opposing the force of the weighing sample, and coil circuit means connecting said servo circuit means and said coil means for applying said current signal to said coil.   
     
     
       8. The balance device of claim 7 wherein said coil circuit means includes circuit means for providing an analog output signal representative of said direct current signal. 
     
     
       9. The balance device of claim 8 wherein said coil circuit means includes further circuit means for adjusting said analog output signal. 
     
     
       10. The balance device of claim 9 wherein said circuit adjusting means includes circuit means for thermal compensation and circuit means for manual calibration. 
     
     
       11. The balance device of claim 10 wherein said measuring circuit means is coupled to said output analog signal. 
     
     
       12. The balance device of claim 11 wherein said measuring circuit means comprises an analog to digital converter.